Symmetrical bridge for reducing hum in vacuum tubes



July 16, 1957 J. J. HUPERT 2,

SYMMETRICAL BRIDGE FOR REDUCING HUM IN VACUUM TUBES Filed July 20, 19532 Sheets-Sheet l A conl lhukous bed 94 IN VEN TOR.

Jigw Jflgper i July 16, 1957 SYMMETRICAL BRIDGE FOR REDUCING HUM INVACUUM TUBES Filed July 20, 1953 6 f/ODULA T10 J. J. HUPERT VOLT/76E 2Sheets-Sheet 2 /000 c p5 =5 PP I I I i Q 50 60 7Q 05 JNVENT Q v Ja/ZusfJlgp'eri SYh IMETPJCAL BGE FOR REDUCING HUM IN VACUUW'TUBES Julius J.Hnpert, River Forest, llL, assignor to A. R. F.

roducts, Ind, River Forest, Ill., a corporation of iiiinols ApplicationJuly 2%), 1953, Serial No. 369,001 13 Claims. (Cl. sis-res Thisinvention is concerned generally with the elimination of hum fromamplifiers, and is concerned more particularly with a circuit forreducing line frequency hum in indirectly heated, sensitive vacuum tubestages.

When low level electrical potentials are amplified to substantiallevels, hum elimination becomes a major problem. The most difiicult typeof hum to jeliminate is that originating in the A. C. heater supply. Humgenerally can be satisfactorily eliminated from plate and grid supplysources by adequate filtering.

A major source of hum and the source which produces the dominating humeffect in sensitive amplifiers and modulators using conventionalreceiving type tubes is the hum caused by leakage between the .cathodeand the heater in indirectly heated tubes. The present invention isconcerned with the elimination or reduction of hum produced from .thissource. I

An object of this invention .is to .provide means for reducing hum inindirectly heated, sensitive vacuum tube stages.

Another object of this invention is to provide means for reducing humcaused by leakage between cathode and heater in circuits employingvacuumtubes having indirectly heated cathodes.

A more specific object of this invention is to provide a bridge circuitfor reducing bum in circuits employing vacuum tubes having indirectlyheated cathodes.

Yet another specific object of "this invention is to provide a bridgecircuit for reducing hum in circuits employing vacuum tubes havingindirectly heated cathodes wherein the balance of the bridge is not.critical and is adjustable.

Still another object of this invention is to provide a circuit forreducing hum in amplifying .circuits employing vacuum tubes havingindirectly heated cathodes in which the cathode has a bias impressedthereon.

tates Patent A further object of the invention is to provide a bridgecircuit forreducing hum in amplificationcircu'its employing vacuum tubeshaving indirectly heated cathodes, the cathode having a bias impressedthereon and being connected directly to the bridge circuit.

Other and further objects and advantages of the present invention willbe apparent from the following description when taken in connection with'the accompanyingdrawings wherein:

Fig. l is .a circuit diagram illustrating the principles of myinvention;

Fig. 2 is a bridge circuit diagram equivalentto :the circuitof Fig. 1;

"Fig. '3 is an equivalent circuit symmetrical bridge for circuitequations;

Fig. 4- is a simplified equivalent circuit for nonlinear leakage; 1

Fig. 5 is a circuit diagram incorporating the principles of my inventionfor testing signal-tmhum ratio; .and

Fig. 6 illustrates the statistical distribution of a num- -ber of tubestested for signal-to-hum ratio inthe circuit of Fig. 5.

"ice

Leakage between cathode and heater is most often caused by contaminationof the heater-cathode space and bordering surfaces by the emittingmaterial. This physical character of the leakage generally brings aboutleakage of a nonlinear-character. However, the leakage may be linear. Itmay be localized or distributed, and it may be symmetrical orasymmetrical (uni-directional).

Grounding of one side of the heater transformer winding has provedunfeasible as a means fo reducing hum. If cathode return bias(self-bias) is used, spurious line voltage is injected into the gridcircuit of the tube as a result of voltage drop across the cathodereturn resistor caused by the leakage current between heater andcathode. The cathode return resistor might be by-passed but in generalthe capacitor required is too large to fit in the space available and isnot economical. The provision of fixed bias is unsatisfactory in view ofthe normal variations to be found among vacuum tubes. There is noautomatic compensation, and a fixed bias suitable for one tube samplemay not work for the next. Furthermore, fixed bias often leads todlfi'lCtlll; component choice or unnecessary design complication.

A grounded center tap on the heater transformer offers some degreeof humreduction, and is the expedient commonly adopted in practice. Thishalves the electrometive force causing the leakage current andeliminates spurious cathode current between the center of the heater andthe cathode. However, this expedient is not satisfactory where extremeimmunity from-hum is desired due to the effect of leakage capacitancesbetween ground and the heater transformer windings. These capacitancesare generally unequal, thus preventing full symmetrizing, and arelargely unpredictable.

I have found that the circuit shown in Fig. l substantially eliminateshum Without possessing any of the drawbacks of the prior art. Thiscircuit is illustrated with a vacuum tube 19 which may be of the triodetype, or orany other type having at least a plate 12, control grid 14,an indirectly heated cathode 16 and a heater is.

The opposite ends of the heater or filament 18 are connected by suitablewires 20 and 22 to the opposite ends 24 of :thefilament winding 26 ofthe transformer 23. It will be understood that the transformer 2S mayincorporate additional windings (not shown) according to the usualpractice in radio power transformers. A pair of capacitors 30, labeledCs on the drawin s, is connected between the ends 24, 24 of the heaterwinding 26, and .the junction between these two capacitors .is groundedas at 32. A resistor 34 is connected between the winding ends 24 inparallel with the series arranged capacitors 30 and is provided with asliding tap 36. The resistance to the left of the tap is indicated at RWhile the resistance to the right of the tap is indicated at R2.

.-A resistor 380i" high value and indicated at r on the drawings isconnected to the sliding tap 36, and is in -turn connectedby a wire it)to the cathode i6 and to a Wire42. The wire 42 is connected to a cathodereturn resistor 44, also labeled as r Leakage resistances between theheater 18:and cathode 16 are indicated at i, 2, and 3.

It is to be noted that the connection between the bridge and the cathodeis not at ground potential but is somewhat above ground potential, asdetermined by the cathode return resistor 44. This circuit arrangementimposes the correct enforced potential on the cathode at all timeswithout interference of hum reduction, as will be more fully describedbelow.

The capacitors 'Cs and the resistors R1 and R2 form a bridge, and it isapparent that when the bridge is balanced there will be no leakagecurrent through the resistors r and r The capacitors in practice are lowtolerance paper condensers, and also include stray wiring capacitances.Thus, .it is apparent that in general they uneven, asymmetricaldistributionj heater length.

i will not be exactly equal, and that for balance of the bridge theresistors R1 and R2 will be unequal.

Critical bridge adjustment is undesirable and hence an approximation ofthe leakage current through the cathode resistorswith the bridgeunbalanced is in order as giving an indication of how criticaltheadjustment is. Calling -the cathode resistance Rcand assuming R =r,-then 'r =O. Referring to the equivalent'circuit of Fig.2,

the equivalent leakage resistance ,between cathode and heater isindicated at Rx (assumed linear and capacitive).

. This resistance can be transferred to one side of the bridge forpurposes of analysis. The transfer of the leakage resistance to one sideof the bridge is indicated in the equivalent circuit. of Figure 3. InFigure 3, Rb'co'rresponds to R1 and R2 in Figures land/2. By rneshanalysis we In the above equation Zr'is imaginary if it represents acapacity.

In a conventional hum reducing circuit wherein the only compensationcomprises grounding the center tap of the heater transformer windingthe'current i flowing through the cathode resistance Re is:

If the resistance Re is made very large, i. e., Rb

i al- Thus, it readily may be seen that the cathode leakage current isgreatly reduced from that present in conventional circuits when thebridge resistance, Rb=R1-{-R2,'

is very large and when {21! is very large.

' Also, when Rb/RfiO, [z',]+0. v Therefore, theratio I Rb/RI desirablyshould be held as small as. possible.

For example, when Rb ='0.1Ra:,

l t: i

192 4 2113,. 7 This means that [i is muchless than i particularly when[Z1[ is of the same order of magnitude as Rx.

(2) RC and the impedance between ground and the equivalent ground pointon the'heater are negligible 1 compared to the nonlinear leakageimpedance, i. e., the

leakage impedance controls the current through RC.

(3) Uniform linear potential distribution across a heater of negligibleimpedance and finite size.

(4) Leakage current density 'i is nonlinear (with respect to thepotential (2 causing it) and is of continuous,

i is defined as current in microamperes' per unit of e is the potentialof a cathode point with respect to ground and is variable with timeand-position.

' 4 Therefore, i;f (e, h)

e=(t,h) namely h 5 7 e- E cos wt Thus 10 V H r z 7 2 h V, l =f Htdh=f fE cos wt,h)dh p 5 p I V i. r Fourier analysis of the periodic functionof time,'I(t 10 reveals values of harmonics.

By Maclaurins theorem:

' where a (h) are functions of h such that "mp f anhv 7fl[r b e=0 Hence:

2 En 1 :7?) ga,,( h) h cos wtdh 71.= n g cos" wiJLEG UQh dh For the caseof step-wise distributed S nonlinear leakage points, the current-timefunction I V n=eeE p=si I 2 eos'f wt2a,,(p)h(p) V n=0 p =l Where ,,(p)'n (p) ar point'set functions of p,

denoting sequential number of any leakage position. For the case ofevenly distributed nonlinearity:

for neven,

z EnHn-H n t .=2,4,6,etc. Dan cos m From the foregoing, it may be seenthat if the nonlinear leakage isdistributed uniformly only evenharmonics will be noticed in the hum spectrum. It easily will be noticedthat this also is true for uneven, but symmetrical distribution. i 1

The bridge circuit shown and described heretofore substantiallyeliminates the fundamental component, which is the major component, butcannot eliminate the harmonic components asflong as'the resistance Rb islinear. :Residual hum after the balancing of the bridge depends on theextent of the nonlinearity. In order to remove'the nonlinearity'eff ect,an additional bias between cathode and heater may be desirable;

Thehum reducing bridge circuit herein disclosed has been testedexperimentally on a large number of tube :amaeoe samples. Conventionalcircuits were found experi mentally to give widely divergent results asto signal to hum ratio with successive .tuhe samples. However, thebridge circuit herein shown and described gave remarkably consistentresults.

The test circuit is shown in Fig. 5 and is similar "to the circuit ofFig. 1 with the addition of an audio frequency voltage from any suitablesource 46. The tube 19 used was 6AH6 reactor tube. The capacitors -eachwere 0.5 microfarad and the resistor 34 was 5000 ohms. The cathodereturn resistor 44 was 100 ohms and the tap resistor '38 was 100,000ohms. One hundred tubes were tested with a60 cycle per second heatervoltage and with a 1000 cycle per second heater voltage.

As may be seen in Fig. 6, with a 60 C. P. 'S. heater voltage all of thetubes produced a signal to noise ratio of between 56 and 64 db, thegreat majority falling between 60'and 63 db. With a 1000 C. P. S.heater'voltage, all of the tubes produced a signal to noise ratio ofbetween 57 and 62 db, most of them being between 59 and 62 db.

The bridge circuit herein shown and described largely eliminates humfrom indirectly heated cathode type vacuum tube stages. The circuit iseasily balanced and is not critical in balance. The circuit iseconomical and produces remarkably uniform results from one tube sampleto another. Most particularly, the hum caused by leakage due tocontamination of the heater cathode space is largely eliminated, andthis hum has heretofore been the most troublesome encountered withconventional receiving type vacuum tubes.

The specific example shown and described is illustrative only, and theinvention will be understood as including all that which falls withinthe spirit and scope of the appended claims.

I claim:

1. A hum reducing circuit for use with tubes having a cathode and aseparate heater therefor, comprising a source of pulsating current forenergizing the separate heater, a pair of capacitors series connectedacross said source, resistance means connected across said source inparallel with the pair of capacitors, a tap on said resistance means, acathode return resistor connected between the cathode and the junctionof said capacitors, and impedance means connected between said cathodeand said tap to provide a relatively low impedance in parallel with theleakage impedance between said cathode and said heater and to provide arelatively high impedance in series with said cathode resistor.

2. A hum reducing circuit for use with tubes having a cathode and aseparate heater therefor, comprising a source of pulsating current forenergizing the separate heater, a pair of capacitors series connectedacross said source, resistance means connected across said source inparallel with said pair of capacitors, an adjustable tap on saidresistance means, a second resistance means connecting said tap to thecathode, and a cathode return resistor connected between the cathode andthe junction of said capacitors.

3. A hum reducing circuit for use with tubes having a cathode and aseparate heater therefor, and including a som'ce of alternating currentfor energizing the separate heater, a pair of capacitors seriesconnected across said source, the junction between said capacitors beinggrounded, resistance means connected across said source in parallel withthe pair of capacitors, an adjustable tap on said resistance means, acathode return resistor connecting the cathode to ground, and anungrounded impedance means connected between said cathode and said topto provide a relatively low impedance in parallel with the leakageimpedance between said cathode and said heater and to provide arealtively high impedance in series with said cathode resistor.

4. A hum reducing circuit for use with vacuum tubes -having1azcathodeand a separate heater therefor, andineluding a source of alternatingcurrent for energizing the separate vacuum tube heater, a pair ofcapacitors series connected across said source, the junction betweensaid capacitors being grounded, resistance means connected across saidsource in parallel with the pair of capacitors, an adjustable tap onsaid resistance means, ungrounded resistance means connecting said tapto the vacuum tube cathode, and a cathode return resistor connecting thecathode to ground.

5. A hum reducing circuit for use with vacuum tubes having a cathode anda separate cathode heater comprising .asource of alternating current forenergizing the separate cathode heater, a pair of capacitors seriesconnected across said source, means grounding the junction between saidcapacitors, resistance means connected across said source in parallelwith said capacitors, an adjustable tap on said resistance means, arelatively large resistor connecting said tap to the vacuum tubecathode, and a relatively very small cathode return resistor connecfingthe cathode to ground.

6. A (hum reducing circuit for use with vacuum tubes having a cathodeand a separate cathode heater comprising a source of alternating currentfor energizing the separate cathode heater, a pair of capacitors seriesconnected across said source, means grounding the junction between saidcapacitors, resistance means connected across said source in parallelwith said capacitors, an adjustable tap on said resistance means, anungrounded resistor connecting said tap to the vacuum tube cathode, anda cathode return resistor connecting the cathode to ground, saidungrounded resistor being substantially smaller than the leakageresistance between said cathode and said heater and being substantiallylarger than said cathode return resistor.

7. A hum reducing circuit for use with vacuum tubes having a cathode anda separate cathode heater comprising a source of alternating current forenergizing the separate cathode heater, a pair of capacitors seriesconnected across said source, means grounding the junction between saidcapacitors, resistance means connected across said source in parallelwith said capacitors, an adjustable tap on said resistance means, arelatively large resistor connecting said tap to the vacuum tubecathode, and a relatively very small cathode return resistor connectingthe cathode to ground, said connecting resistor being on the order ofone thousand times the value of the cathode return resistor.

8. A hum reducing circuit for use with vacuum tubes having a cathode anda separate cathode heater and comprising a source of alternating currentof a predetermined frequency for energizing the separate cathode heater,impedance means connected across said source and having an intermediatepoint grounded, said impedance means having a relatively very highimpedance at said predetermined frequency, resistance means connectedacross said source and of relatively very high resistance, a tap on saidresistance means, a resistor connecting said tap to the vacuum tubecathode, and a resistor of relatively very low resistanceinterconnecting said cathode and ground.

9. A hum reducing circuit for use with vacuum tubes having a cathode anda separate cathode heater and comprising a source of alternating currentof a predetermined frequency for energizing the separate cathode heater,capacitance means connected across said source and having anintermediate point grounded, said capacitance means having a relativelyvery high reactance at said predetermined frequency, resistance meansconnected across said source and of relatively very high resistance, atap on said resistance means, a resistor connecting said tap to thevacuum tube cathode, and a resistor of relatively very low resistanceconecting said cathode to ground.

10. A hum reducing circuit for vacuum tubes having alcathode and aseparate heater th erefor and comprising a source of alternating currentfor energizing the separate heater, impedance means connected acrosssaid source and having an intermediate point grounded, resistance meansconnected across said source and having a resistance which is smallrelative to the leakage resistance between the vacuum tube heater andcathode, a tap on said resistance means, means connecting said tap tothe vacuum tube cathode, and a resistor connecting said cathode toground.

11. A hum reducing circuit for use with tubes having a cathode and aseparate heater therefor, comprising a source of pulsating current forenergizing the separate heater, a first impedance connected across saidcurrent source, a second impedance connected in parallel with said firstimpedance, a movable tap on said second "impedance, a third impedanceconnected between said movable tap and said cathode, and a fourthimpedance connected between said cathode and an intermediate point onsaid first impedance, said third impedance being small compared to theleakage impedance between said cathode and said heater and being largecompared to said fourth impedance.

12. A hum reducing circuit for use with tubes having a cathode and aseparate heater therefor, comprising a source of pulsating current forenergizing the separate heater, capacitance means connected across saidsource, a first impedance means connected across said source in parallelwith said capacitance means, a movable tap on 0 said first impedancemeans, 'a second impedance means interconnecting said movable tap andsaid cathode, and

a third impedance means interconnecting said cathode and an intermediatepoint on said capacitance mansfsaid said'cathode, and a third resistanceinterconnecting said cathode-and an intermediate point on saidcapacitance, said second resistance beingsmall compared to theleakage'resistance between'said cathode and saidheater and being largecompared to said third resistance.

References Cited in the tile of this patent V V UNITED STATES PATENTS1,743,629 Spaeth Jam 14, 1930 1,896,534 Alexanderson Feb. 7, 19331,959,156 Downey May 15, 1934 1,983,802 Miessner Dec. 11, 1934 2,078,666Ka Dell Apr. 27, 1937 Lindenblad Dec. 10, 1946'

